Debug Types Sections

Summary

DWARF 4 added a new ".debug_types" section. This section enables a relatively simple and cheap way to compress DWARF debugging information. This feature changes Fedora to take advantage of this compression.

Owner

Current status

Detailed Description

This feature works by enabling a compiler option that instructs GCC to put "large" types into comdat sections in the object files. Then, the linker automatically merges all such shared types when linking. Since types occupy a reasonable proportion of the DWARF data, and because types are often identically across compliation units, this results in space savings.

The .debug_types feature was implemented in GCC and GDB quite a while ago, but until now Fedora has not taken advantage of it. The compression is quite good. For exmaple, building gdb with this option reduces the size of the final executable from 17M to 11M. A stripped gdb is 6.7M; meaning that this flag removes around 60% of the debuginfo.

Benefit to Fedora

This reduces the size of debuginfo by quite a bit, just by enabling a compiler flag.

Scope

Several changes are needed to integrate this change into Fedora:

Either change GCC to enable -fdebug-types-section by default, or add this to the default compiler flags used when building RPMs.

Update SystemTap to handle the new section properly. (In stap git repository now; must ensure it is in the stap that is shipped in F18.)

Update the dwarves package to handle the new section properly. ([Patch submitted])

Contingency Plan

Documentation

Release Notes

Packages built in Fedora 18 are compiled with -fdebug-types-section, yielding much smaller debuginfo packages. The new debuginfo is specified by the DWARF 4 standard; some debuginfo-reading packages may need updates in order to understand the new data. All DWARF-reading packages in Fedora 18 have been updated.